Party-line telephone system



Oct. 17, 1939. B. KUDRNA PARTY-LINE TELEPHONE SYSTEM Filed Oct. 7, 1936 3 Sheets-Sheet 1 INVENTOR. BE IKT KUDRNA ATTORNEY? N ww EN 5% 3 a :5 Ex 1% \ma L I .k 2% \I m :5 E a s H m 8 HH L AL 3 a5 2 d Oct. 17, 1939. B. KUDRNA PARTY-LINE TELEPHONE SYSTEM Filed Oct. 7, 1 936 3 Sheets-Sheet 2 INVENTOR. KUDRNA BENEDIKT ATTORNEY. I

Patented Oct. 17, 1939 UNITED STATES PATENT OFFIQE PARTY-LINE TELEPHONE SYSTEM Application October 7, 1936, Serial No. 104,374 In Austria October 28, 1935 15 Claims.

The invention relates to a circuit arrangement for selective calling in remote communication systems and more particularly in telephone systems having party lines.

5 The invention aims at efiiciently selecting a party line from a large number of branch stations without using switching devices or selecting de- Vices tuned to alternating currents of particular frequencies at the branch stations. 7

10 According to the invention this is achieved in that selecting currents of various durations and in differentdirections influence condenser-reslstance arrangements at the branch stations of the party line, and condenser currents dependent,

l5 as regards their direction and strength, upon the selecting currents operate the connecting means of the branch stations characterized by the selecting currents.

By the arrangement according to the invention 20 it is possible to effect the selection, even from a large number of branch stations, by one application of current for a relatively short duration. If the number of branch stations is increased by any multiple, two successive applications of current sufiice in general Arrangements of the type according to the invention can, since no sources of current are necessary at the branch stations, be utilized especially in those systems in which the question of current 30 economy at the branch stations of the party lines is of importance.

The condenser-resistance arrangements can either be used as shown in the embodiment in that the condensers, which are not charged in the rest condition of the system, are charged over high resistances during a particular period and then discharge over the connecting relays, the connected means at the branch stations being operated according to the direction and strength of the discharging current, or alternatively in that the condensers which are charged in the normal condition of the system are discharged to a particular level over high resistances during a particular period and then charged up over con- 45 necting means, in this case the switching means of the branch stations being operated in accordmice with the direction and strength of the charging current.

One embodiment of the invention is represented in the attached drawings, comprising Figs. l to 5.

The invention is not restricted to this embodiment.

Fig. 1 shows party line GL having several branch stations which are adapted to be called 55 selectively from an exchange station ZSt;

Fig. 1a shows the exchange station ZSt and the associated key sender KS for selectively calling the branch stations;

Fig. 2 is a chart showing the polarity and relative strength of the selecting currents; Fig. 3 is a chart showing the relative states of charge imparted to the condensers by the selecting currents;

Fig. 4 is a fragmentary circuit drawing illustrating the manner of group selection; and

Fig. 5 similarly illustrates the manner in which the wanted station is selected from those of a selected group of stations.

Referring now particularly to Figs. 1 and la, the branch stations are divided into groups, the group selecting devices GM to GT6 either being provided in common for all the individual branch stations of a group or separately for each individual branch station. Inter-communication between the individual branch stations is possible but all calls necessitate the cooperation of the exchange. Call secrecy is ensured. Simple local battery magneto telephones are used at the subscribers stations.

The selection of an individual branch station is effected by direct current impulses of a particular duration and direction. The use of, for example, three different lengths of impulses and two different directions gives six different call characteristics (see Fig. 2). If these basic elements are used both as group and individual calling signals they provide 36 different possibilities of calling. This number can in general be reached. It can, however, be increased by using further basic elements.

The switching operations of the arrangements shown in Figs. 1 and 1a are as follows:

In the normal condition of the system potential is applied to the a-lead of the party line by the exchange. If now a subscriber, for example at station TStl 1, takes up his receiver and operates his magneto, relay Al I is energized at the branch point associated with this subscribers station; relay All (winding I) contact ZSpl and earth. Relay AH closes its contacts 3aH and tall and opens contact 5a! l. Through the closing of 3a a circuit is established for relay X in the ex-- change: earth, battery, relay X (winding I), contact 6e, relay X (winding II) contact 1:13, a-lead lead of the party line, contact 88291 in the group station (Grl), contact 3a! I, relay All (Winding I) contact Zspl and earth. Relay Al I holds up in this circuit in the exchange. Negative potential is applied to the b-lead over contact 9x through the energizing of relay X, while the negative po- 55 the b-lead of the party line.

tential is disconnected from the a-lead by the opening of contact ix. Furthermore the buzzer Sit is switched on over contact Illa: and over its winding II and condenser Kozl applies tone to the a-lead of the party line so that the calling subscriber is notified when the party line is free and the call has reached the exchange. By means of the magneto current transmitted by the subscriber the indicating flap F]; at the exchange release after the energizing of relay X and op erates as a calling signal.

By the application of negative potential to the b-lead of the party line the relay Sp is energized at each branch point of a group, the relays being energized in the following circuit: earth, battery, relay X (Winding I), contacts 6c, 9r, b-lead of the party line and then in parallel with contact 71, relay Sp to earth and over contact I212, relay S272, etc. The Sp relays (S171, S102, etc.) energize, by opening contacts I3spl, MspZ, [53102, etc., break the starting and connecting circuits for all the subscribers stations of the party line (guarding the line). Contact 2spl is also opened. The circuit of the connecting relay All of the calling station is, however, not interrupted since this relay holds up over its winding II in parallel with relay Spl over contact 4a.! I.

The operator in the exchange who notices the calling signal (indicator flap Fk: down) inserts her inquiry plug in the inquiry jack associated with the calling party line. On plugging in, the indicator flap F70 is switched 011". Furthermore negative potential is applied over the jack contact 7017, resistance Wy'zl and the choke coil Drl to At the same time relay E is switched on. This operates and disconnects relay X from the b-lead of the party line. In addition over contact He and the condenser K022, the connecting path to the jack is held after the release of contact Him. The operator now inquires the call number of the calling subscriber for the purpose of call metering. In order to prevent any wrong charging of subscribers who have not made a call, resulting from false call number being given, the calling subscriber is again called by the operator of the exchange. If,

however, such measures are not necessary the selection of the desired number can take place immediately after the number of the desired subscriber has been stated.

If the calling subscriber is to be called again, the operator at the exchange, after having transitorily withdrawn the inquiry plug and thereby disconnected the negative potential from the 1)- lead of the calling subscriber of the party line, calls the group number of the calling subscriber.

In order to enable selection to be controlled, a key sender KS is provided in common for a number of lines such as GL. This key sender may be connected to the line G-L upon the actuation of transfer key KT, which transfers the a. and b conductors of line GL from the switchboard into association with the key sender KS. The leads a and b of the line GL are transferred through make-before-break contacts to the leads a. and

b" of the key sender to insure the maintenance of holding potential on at least one of the leads.

I It is to be noted that a negative potential is applied upon the operation of the key KT to the a lead of the line GL, as a negative potential is normally impressed on lead a. through contacts of relay T. The application of this holding potential causes actuation of all guard relays Sp to occur to connect up all of the fas p r J relays.

The key sender KS is composed of six selecting digit keys, K1 to K6, together with control relays R, S, and T, and the indicating lamp L. The function of this key set is primarily to place a selecting potential on the I) lead of a. connected party line of a desired polarity and duration. Two polarities of potential are used, which taken in combination with selecting-current durations of 50 milliseconds, 200 milliseconds, and 800 milliseconds, give six selective current conditions over the b lead.

By arranging the station selecting apparatus so that the first pulse selects a group of stations and the next succeeding pulse selects the desired station in the group, the desired one of thirty-six stations on a party line can be selected by two key actuations.

The polarity of the potential applied to the selecting lead is controlled directly by the contacts of the keys, while the duration of the current application is controlled by the time required for relay R to operate, which time depends upon which of the three condensers CI to C3 is connected in shunt of relay R upon the operation of a key.

As indicated by labels associated with the designations of the keys Kl to KS, a current application of 50 milliseconds is made upon either of the keys KI and K2; a current application of 200 milliseconds is made upon the operation of either of the keys K3 and K4; and a current application of 800 milliseconds is made upon the operation of either of the keys K5 and KB. The polarity signs associated with the key designations indicate that the first key of each pair applies negative potential, while the second key applies a positive potential.

It is to be observed that contacts to the left of keys KI to K6 are shown under the label E. These contacts are common to the selecting keys, being arranged to be actuated whenever either of the keys Kl to K6 is operated.

As will be subsequently brought out, in the event that the key operated is any one of the keys K3 to KS, and the charging time permitted is 200 milliseconds or 800 milliseconds, a timed discharge period equal to one-fourth of the charging period is permitted, which allows fully charged condensers which have the next lower charging time to become practically completely discharged, while leaving the condensers whose charging time corresponds to the time of application of the charging current about threefourths charged. The station selecting equipment is arranged tooperate from. a condenser about three-fourths charged.

It is to be observed that each of the condensers CI to C3 is normally short circuited, whereby each of them is in a normally discharged condition.

Since the calling station is station I i, this station is recalled by two successive actuations of the key Kl.

Upon being actuated the key KI opens the discharge circuit of the condenser Cl and connects the condenser to ground, at the same time applying ground potential to the lower terminal of relay R, whereby the relay R and the condenser C! are now connected in parallel with each other. At the same time, the upper group-E common contact applies positive potential through contacts of the non-actuated keys K2, K4, and K6 to the charging conductor 5! Charging current therefore flows from conductor 50 through contacts of relays S and T, over the conductors b and b to'the selecting apparatus of the party line, which flow of charged current continues until relay R becomes operated to terminate it.

The operation of relay R occurs in the circuit which extends from the charging conductor 50 through resistor 3|, contact 32 of relay S, to ground through the winding of relay R and the lower contacts of the key Kl. The condenser CI as above noted, is shunted around the winding of relay R at this time, through contacts of the actuated key K5 and contacts of relay R. The condenser C l is of such capacity that it acts as an effective shunt on the winding of relay R to prevent operation of relay R until a period of approximately 50 milliseconds has elapsed. This period, it will be understood, is somewhat shorter in the event that the exchange battery potential is higher than normal, and is somewhat longer in the event that the exchange battery potential is lower than normal. By this arrangement, the battery-potential variations are automatically compensated for, whereby charging of the condensers on the party line is effected to about the same extent regardless of allowable variations in exchange battery potential.

When relay R operates, it opens a point in the circuit of relay T at contact 37 and closes a circuit for relay S at contact 38. Relay S is operated from the potential applied to conductor 50.

Upon operating, relay S opens the charging circuit at contact 6, and it closes a discharge circuit at contact 34.

Relay S also closes a locking circuit for itself at contact 39 to ground through contacts of the actuated key Kl; it prepares a circuit for relay T at contact 40; and at contact 32 it'opens the previously existing circuit for relay R and the condenser CI in parallel. When this occurs, the condenser Cl starts the discharge through relay R. The resistance of relay R is such that a condenser discharges through the relay in about one-fourth of the time required to charge it. This efiect is utilized in the case of operation of the relay R in association with the higher capacity condensers C2 and C3 to fix the discharge time following the application of charging potential. At this time, however, condenser Cl is the one in use, and no discharge period is required, for a minimum charging time has been used.

When relay R restores, condenser Cl is disconnected by relay S to permit relay R to restore immediately, it closes a circuit at contact 3?, through contact 49 of the locked relay S, for relay T. Relay T locks itself at contact 44 through the common key contact 28 to ground; at contacts 5 and 4| it opens the momentarily closed discharging circuit and places a negative holding potential on lead b through resistor 4.12; and at armature i4 it disconnects potential from the a lead in order to permit the selective switching operations to be subsequently described to take place at the selecting apparatus on the party line.

Also, at armature 43 relay T closes a circuit for the lamp L from the conductor 50, thereby lighting the lamp L to indicate to the operator that she may now restore the actuated key.

Upon noting the lighted condition of the lamp L, the operator releases the key Kl, which she has been holding actuated awaitingthe lighting of the lamp L. When the key Kl is released, the local discharge circuit of the condenser Cl is again prepared, and potential is removed from the charging conductor 59, permitting the lamp L to become extinguished and relays S and T to restore. Condenser Cl discharges when relay S closes its inner lower contacts. The restoration of relays S and T is further assured by the opening of the locking circuit of relay S at the lower contacts of key KI and the opening of the locking circuit for relay T at contact 28.

With the relays R, S, and T all restored, and with the condenser Cl again discharged, the key sender KS is again in normal condition, and negative potential is again applied through the common a lead to the a lead of the party line.

In order to transmit the current impulse representative of the second digit 1 in the number of the substation TStll, the operator again actuates the key Kl for a sufficient length of time to bring about the lighting of the lamp L and then restores it, the results being the same as those described.

Following selection, the operator restores key KT.

It is to be noted that condenser C2 is permitted to remain connected in shunt of relay R after relays R and S operate to mark the end of a 200 millisecond charging period (keys K3 and K4), whereby relay R is held operated by discharge current from condensers C2 for about 50 milliseconds.

Similarly condenser C3, by its discharge current, holds relay R operated for about 200 milliseconds following an 800 millisecond charging pulse (keys K5 and K6).

When the 50 millisecond positive charging potential is applied to the I) lead the first time, the J relays energize at all the group branch points; for example, relay J I over contact 223p! relay J2 over contact 23sp2, etc. Accordingly, the contact Hg! and 1272, etc., are brought into the operating position. The circuit for relays Spl and S102 passing over these contacts is not broken since these contacts are constructed as make-before-break contacts and in their working position connect the relays Spl, S322, etc., in parallel to the relays J l, J2, etc., to the a-lead of the party line to which negative potential is applied. By the energizing of relays J i and J2, the condensers Kol, K02, etc., in all the group branch stations are charged over resistances Wy'l, W7'2, etc., and contacts 247'! and 25,42, etc., in correspondence with the transmitting period of 50 milliseconds. Since either the resistances at the individual group branch stations or the capacities of the condensers, or both together, are adjusted so that the condensers of the group branch stations GM and GrZ are 100% charged after 50 milliseconds the condensers at group branch stations 3 and 4 only after 209 milliseconds and the condensers at group branch stations 5 and 5 only after 800 milliseconds, only the condensers of groups G44 and GT2 are 100% charged after the lapse of 50 milliseconds.

The guarding Sp relays, however, are weakly tensioned, whereby they do not restore as quickly as the J relays. The Sp relays are held operated through contacts such as Hit and I2"? over the b lead, to which negative potential is new connected through resistor 42.

Upon the release of the J relays, the condensers charged over the resistances are thus discharged over the winding I of group connecting relays Gl, G2, etc. Only those G relays at the group branch stations Gr! and GM become energized. The group characterizing relays G, which are constructed as two-stage relays, respond to the .condenser discharge current by energizing only to the first stage.

In the group branch station GTI an energizing circuit is established for relay GI (Winding II) in parallel with the guarding relay Sp! by the switching over of contact 29gII (first stage) in the operating position over contact 3IlgIII (second stage) which is still in the rest position. In the same way relay G2 is energized in parallel with the guarding relay 8122 over its winding II and contacts 3| 921 (first stage) and 3247211 (second stage). Since the condensers were charged by a short impulse in the positive direction the discharge current at the group branch stations GTI, GT3, and GT5 flows through the winding I of the group characterizing relays GI, G3, and G5 in the same direction as the energizing current from the windings II of these relays, i. e., the two windings are supplementary in their effect (see Fig. 4). Since only relay GI of the relays GI, G3, and G5 was energized only this relay remains energized and holds up over its winding II; through the closing of contact 33951 (stage 1) a circuit is prepared for winding I of relay GI.

At the group branch stations GT2, GT4, and GT5 the discharge current from the condensers flows through the windings I of the group characterizing relays G2, G4, and GB in the opposite direction to the current in the energizing windings II (see Fig. 4) Relay G2, which on account of the short-charging period of the condenser was the only group characterizing relay of the groups 2, 4, and 6 to be connected up, is disconnected by the influence of the opposing efiects of the currents in windings I and II and accordingly returns to the rest position.

Thus at this time the only group relay energized is relay GI at the group station GTI, it being operated only to its first stage of operation.

On the described release of relay T of the sender, and the consequent transfer of negative potential from conductor b back to conductor a, the J relays again operate. The energizations of the J relays occur to switch the circuit of the Sp relays over to lead a before the Sp relays and the partially operated relay GI can restore responsive to the removal of negative potential from the I) lead.

In the group branch station Gr only is a circuit set up for Winding I of relay GI over contact 'SIigII (first stage) which is the only one of these contacts in the operated position. Since winding I of this relay is an efiicient winding designed for strong energization, relay GI energizes completely, drawing up this armature fully, and is maintained energized in the second stage. Accordingly contact MglII (second stage) is closed.

Upon the next key operation, the positive potential of the b-lead of the party line is connected through contact 349111 to all the condensers of the individual branch stations AstII to ASt'IE which are connected at the group station GrI, the potential being supplied to the condensers over resistance Wy'II, W712, etc., and contacts 351i, esp, etc. The condensers KoI I, KOI2, etc., therefore, charge up during the transmission of the impulse. The condenser-resistance arrangements at the individual branch stations are adjusted to possess different charging periods in the same way as at the group branch stations. Since only a short impulse lasting 50 milliseconds is in question only the condensers K0! I and KoI2 become 100% charged. When, therefore, the second pulse is terminated and the negative potential is replaced on conductor 1) and is removed from conductor a, the charged condensers Kol I and Kol2 commence to discharge as soon as relay J I releases and allows contacts 357 I and 367'! to move into the rest position.

The discharge current of condenser KoII for branch station I I flows through the associated relay All and causes this to energize so that contacts IaI I, Sal I, flal I, and 5a. are switched over into the operating position, and are held by current over the b lead through winding AIII. The direction of the current flowing from the 17- lead of the party line over contacts II I and 3'lal2 through winding II of relay AI2 is, however, opposite to the direction of the discharge current from the condenser so that the relay AI 2, just energized, falls back again (see Fig. 5). Thus at the end of the individual selection only the connecting relay All at station AStI I is energized so that only this subscribers station TStII is connected over contacts IaI I, MI I, and dull to the common line.

The operator at the exchange now calls the selected subscriber at substation TStII in the known way by means of alternating current. When the subscriber replies the operator knows that only this subscriber is connected to the party line. She can now establish the connection to the desired subscriber, for example another subscriber of the same exchange. The operations which are necessary for selecting a subscriber on the same line will be further explained later.

If the call does not proceed from the subscriber, at station TStIl but, for example from the subscriber at station TSt22, the fundamental establishment of the call is the same. The differ ence is that two 50 millisecond negative pulses are transmitted instead of positive pulses. The condensers this time are charged over the resistances by negative potential. When the transmission of the first impulse is finished and the condensers discharge after the release of the relays J, only relays GI and G2 energize in the group branch stations G1"! and GT2 since the condensers at the other group branch stations were not sufliciently charged on account of the short duration of 50 milliseconds of the impulse. Windings II of the relays GI and G2 are again connected to the I)- lead of the party line which is at a negative potential, so that the current flowing through" these windings corresponds in direction with the discharge current from the condensers flowing over the windings I, only at the group branch station GT2, so that relay G2 remains energized in the first stage and closes its contacts 4Iig2I (first stage, Fig. 4). In the group branch station GrI the discharge current from the condenser is in opposition to the energizing current flowing through winding II of relay GI so that relay GI releases (see Fig. 4). Thus after the group selection relay G2 only is energized in the first stage at the group branch station GT2. When the transmission of the negative individual selecting impulse is effected and terminated, the charged individual selecting condensers discharge over relays A2 I-A22. Since the condensers were only charged during a period of 50 milliseconds, only the discharge current from individual selecting condensers K02I and K022 is sufficient completely to energize relays A2! and A22. Both relays energize and close their contacts MaZI, 45ml, 46a2I, MaZI, and 48a22, 59L222, 501122, and 5Ia22. Accordingly current flows through the b-lead of the party line both over winding II of relay A2I and over Winding II of relay A22. This current is in the same direction as the discharge current from condenser K022 so that relay A22 remains energized (see Fig. 5), while the current flowing over winding II of relay A2 I- is opposite in direction to the discharge current flowing from condenser K02! so that this relay releases (see Fig. 5). Thus only subscribers station TSt22 is connected to the party line.

When selecting a subscriber of the group branch stations GT3 or GT4 an impulse of, for example, 200 milliseconds duration, is transmitted. In such a selecting operation naturally the condensers K01 and K02 of the group branch station Grl and GT2 arecompletely charged. In order to enable an efiicient selection of the group branch station G 3 or GM in spite of this and to prevent the group characterizing relays G l and G2 from energizing, all the charged condensers KoIKo5' are again discharged over the b-lead of the party line for a period of 50 milliseconds by controlling the key sender KS as described. The condensers K! and K02 which are completely charged when the charging period amounts to 50 milliseconds are completely discharged while the charge on condensers K03 and K04 in the group branch stations Gr3 and Gr i is still suf icient (about 80% of the complete charge) to operate eificiently the relays G3 and G4 arranged in series with the condensers, when these discharge. The same operations are repeated in a corresponding way during the individual selection.

If one of the subscribers associated with the group branch stations 5 or 6 is to be connected, an impulse of for example 800 milliseconds duration is transmitted. During this period the condensers at all the branch stations are completely charged. In order to prevent the group relays GiG fromenergizing all the condensers are discharged again over the b-lead of the party line for a period of 200 milliseconds. Accordingly the condensers Kai-K04 in the group branch stations Grl-Gr4 are completely discharged, while the charge on the condensers at the group branch stations 5 and 6 still suflices (see Figure 3), to operate group relays G5 and G6 on the final discharge of the condensers.

In the switching operations already described only that case has been considered in which a subscriber of the party line calls the exchange and is recalled from the exchange for control purposes. The operations which take place when a subscriber ofthe party line is to be called in an incoming call, for example by another subscriber of the exchange, are however, the same as those already described in the case of a call proceeding from the operator at the exchange.

If a call is to be set up between two subscribers of the same party line (internal call) no difficulties are involved. After the calling subscriber has been selected in the manner already described and has been connected to the party line (by the energizing of his connecting relay A) the selection of'the desired subscriber is effected by the renewed operation of the key set in correspondence with the call number of the desired subscriber. The connecting relay of the first subscriber is not influenced in any way for this is arranged in parallel with the guarding relay Sp and like the latter is connected over the makebefore-break contacts (for example, Hy'l, I272, or the like) according to the switching condition to negative potential over the a-lead or the b-lead.

Since the connecting relay of the subscriber who has once been connected to the party line is not influenced by further switching operations during the existence of a call it is possible if desired to connect a number of subscribers of a party line to the latter together for example for the purpose of a conference call.

The release of the call takes place when one of the subscribers operates his magneto at the end of a call and transmits a clearing signal to the exchange by means of the alternating current produced thereby. As soon as the operator withdraws the plug from the jack of the party line the negative potential previously connected to the b-lead of the party line is disconnected. The guarding relays Spl, S 02, etc., and those of the group characterizing relays GI, G2, etc., which are energized thus release as do also the subscriber connecting relays All, Al2, etc., in the individual branch stations, so that the whole system is once more in the normal condition.

What is claimed is:

1. In a telephone system, a central office, a line terminating in said ofiice, a plurality of branch lines connected to said line, a plurality of stations accessible to each of said branch lines, means controlled from any one of said stations for completing a connection over said line and one of said branch lines between a calling station and said central office, and means connected to each branch line operated from said central office over one of the conductors of said line whenever a connection is extended from a station for preventing any other station from connecting with any of said branch lines.

2. In a telephone system wherein a common line is provided having a plurality of branch lines and wherein each of the branch lines is common to a plurality of station lines, a central ofiice connected to one end of said common line, means at said central office for transmitting selecting currents of different duration and direction over said common line, a condenser-resistance coupling connected to each of said branch lines controlled by said selecting currents, connecting means connected to each of said branch lines for connecting the station lines associated therewith to said common line, and condenser currents transmitted by said couplings for operating only the connecting means selected by the particular selecting currents transmitted from the central oflice.

3. In a .telephone system wherein a common line is provided having a plurality of branch lines and wherein each of the branch lines is common to a plurality of station lines, a condenserresistance coupling unit connected to each of said.

branch lines, connecting means associated with each of said branch lines for connecting its associated plurality of station lines to said common line, a central office connected to one end of said common line, means at the central office for transmitting selecting currents of different duration and polarity over said common line to control said condenser-resistance coupling units, condenser currents transmitted by said coupling units which currents depend upon said selecting currents for their strength and polarity, and means operated responsive to said condenser currents for controlling only the connecting means of the branch line selected by the central ofiice.

4. In a system as claimed in claim 2, in which each of the connecting means is a two-winding two-step relay, one of said windings controlled by said condenser currents to energize said rerelay to its first step, and an energizing circuit completed for the second winding of said relay responsive to its energization to its first step for energizing said relay to its second step.

5. In a telephone system wherein a common line is provided having a plurality of branch lines and wherein each of the branch lines is common to a plurality of station lines, connecting means associated with said common line and individual to each of said branch lines for connecting its associated branch line to said common'line, a central office terminating one end of said common line, means at said central ofiice for transmitting a direct current pulse of particular duration and polarity over said common line to control said branch line connecting means, the particular branch line connecting means operating and connecting its associated branch line to said common line being determined by the duration and polarity of the particular transmitted direct current pulse, a station line connecting means individual to each of said station lines for connecting its associated station line with said branch line, each of the station line connecting means common to the connected branch line and controlled responsive to an additional direct current pulse of particular duration and polarity transmitted from said central office, the particular station line connecting means operating and connecting its associated station line to said branch line being determined by the duration and polarity of the additional transmitted direct current pulse.

6. In a telephone system, a central office, a line terminating at one end in said oflice, two stations normally disconnected from the other end of said line, a condenser unit and a connecting relay individual to each of said stations, means at said central oiiice for transmitting a station selecting pulse of direct current of negative or positive polarity over said line to charge said units, means responsive after the units have been charged for discharging them through their associated connecting relays, one of said connecting relays adapted to be energized to connect its associated station to said line only if the current discharged through said relay from said unit is of a positive polarity and the other of said connecting relays adapted to be energized to connect its associated station to said line only if the current discharged through said relay from its associated unit is of a negative polarity.

7. In a system as claimed in claim 5, in which the selected branch line connecting means is a two-step relay which is energized through its first step by the direct current pulse transmitted from said central ofiice and is energized in its second step to prepare the station line connecting means responsive to the transmission of the station selecting direct current pulse of a particular polarity.

8. In a telephone system, a central oflice, a common line extending from said ofl'ice, a plurality of subscribers stations normally disconnected from said common line, a connecting relay at each of said stations for connecting its associated station to said common line, a condenser associated with each of said connecting relays, means at said central office for transmitting a station selecting pulse of a particular duration and polarity to charge all of said condensers, means at said central office for partially discharging certain of said condensers and for completely discharging certain otherof said condensers, means responsive to the termination of the central ofiice charging and partially discharging operation for controlling the partially charged condensers to completely discharge through their associated connecting relays, one of the connecting relays through which its associated partially charged condenser discharges adapted to be energized only when the condenser charging pulse is of a positive polarity, the other one of the connecting relays through which its associated partially charged condenser discharges adapted to be energized only when the condenser charging pulse is of a negative polarity, and means controlled responsive to the energization of any one of said connecting relays for connecting its associated station to said line.

9. In a telephone system, a central oifice, a common line extending from said office, a plurality of subscribers stations normally disconnected from said common line, a connecting relay at each of said stations for connecting its associated station to said common line, a condenser associated with each of said connecting relays, means at said central office for transmitting a station selecting pulse of a particular duration and polarity to charge all of said condensers, means at said central ofiice for partially discharging certain of said condensers and for completely discharging certain other of said condensers, means responsive to the termination of u the central ofiice charging and partially discharging operation for controlling the partially charged condensers to completely discharge through their associated connecting relays, one of the connecting relays through which its associated partially charged condenser discharges adapted to be energized only when the condenser charging pulse is of a positive polarity, the other one of the connecting relays through which its associated partially charged condenser discharges adapted to be energized only when the condenser charging pulse is of a negative polarity, means controlled responsive to the energization of any one of said connecting relays for connecting its associated station to said line, and means at the central office for transmitting alternating current over said common line to signal only the subscriber at the station connected to the common line.

10. In a telephone system, a common line, a plurality of subscribers lines normally disconnected from said common line, means associated with each of said subscribers lines and controllable from a calling subscribers line for connecting such calling subscribers lines to said common line, means connected to said common line operated responsive to a call from any one of said subscribers lines, and means individual to each of said subscribers lines controlled by the operation of said last means for preventing any other subscriber from connecting his line with said common line as long as it is engaged.

11. In a telephone system in which a plurality of stations are connected to one end of a common line and in which a particular one of said stations can be selected and connected to said line and all other stations can be excluded, a central oflice connected to the other end of said line, a station selecting device thereat comprising a selecting key for each of said stations, a positive current source, a negative current source, means controlled by the operation of said station selecting keys for connecting one or the other of said current sources to said line for a predetermined time interval, the polarity of the current and the length of time it is connected to said line controlling the connecting of the selected station to said line and the preventing of the connecting of all other stations.

12. In a telephone system, a central office and a common line therefor, a plurality of stations connected to said common line, a sending device located at said central oifice, a condenser-resistance unit for said device, a condenser-resistance unit for each of said stations, a source of current at said central ofiice, means including said device for connecting said source of current to said common line to charge said station condenserresistance units and to charge said sending device condenser-resistance unit to select one of said stations, and means controlled in accordance with the time interval required to charge said sending device condenser unit for disconnecting said source of current from said common line.

13. In a telephone system as claimed in claim 12, in which the time interval required for charging the sending device condenser-resistance units determines the station selected by the operation of said device.

14. In a telephone system, a party line, a central office connected to one end of said line, subscribers stations normally disconnected from the other end of said line, a condenser for each of said stations, a relay at each of said stations for connecting only its associated station to said line responsive to discharged current of a particular duration and polarity from its associated condenser, a station selecting device at said ofiice comprising a station selecting key for each of said stations, means responsive to the operation of one of said keys for applying current of a particular polarity to said line to charge all of said condensers, means at said office for automatically disconnecting the applied current after a predetermined elapsed time interval, the time interval being determined by the station selecting key that is operated, and means at said stations for controlling the condensers to discharge through their associated relays.

15. In a telephone system wherein a common line is provided having a plurality of branch lines and wherein each of the branch lines is common to a plurality of station lines, a condenser-resistance coupling unit connected to each of said branch lines, connecting means associated with each of said branch lines, a central oflice connected to one end of said common line, means at the central oflice for transmitting selecting currents of different duration and polarity over said common line to control said condenser-resistance coupling units, condenser currents transmitted by said coupling units which currents depend upon said selecting currents for their strength and polarity, means operated responsive to said condenser currents for controlling only the connecting means of the branch line selected by the central ofiice, each of said connecting means comprising a two-winding two-step relay, one winding of said relay connected to its associated condenser-resistance coupling unit after the transmitted selecting current has been received by the coupling unit to energize said relay in its first step by the condenser currents discharged from said unit, and said relay adjusted to energize to its second step only when the condenser current discharged by said unit is of a particular polarity.

BENEDIKT KUDRNA. 

